1. Field of the Invention
The present invention relates to aqueous binders based on polybutadiene which are suitable for use as air-drying coating materials and which can be processed entirely without organic solvents.
2. Description of the Background
Aqueous coating systems have become increasingly important as they enable reductions in solvent emissions.
As a consequence of the continual increase in environmental awareness, tighter statutory regulations and the introduction of so-called eco audits, the necessity of maintaining clean air continues. As a result, producers and users of coating systems must develop and employ processes and products devoid of harmful, environmentally polluting emissions.
Aqueous binders based on polybutadiene are obtainable by modification with maleic anhydride (MAA), wherein the resulting anhydride functions are reacted further to afford free carboxyl groups. Partial or complete neutralization of these carboxyl groups provides water-soluble or water-dispersible binders.
The known modification of polybutadienes with MAA is described, for example, in DE-B 12 92 299 and in DE-B 12 19 684. Films of such MAA adducts show improved substrate adhesion compared with those having polybutadienes which have not been modified.
Polybutadiene/maleic anhydride adducts can be partially esterified or amidated; see DE 10 26 900, 25 07 809, 26 33 362 or 27 33 274. The monoesters or monoamides formed, respectively, in the reaction with alcohol or with secondary amines have free carboxyl groups. Some or all of these groups can be neutralized, thereby giving water-soluble or water-dispersible binders, respectively.
It is also possible to obtain water-soluble or water-dispersible binders by imidating the anhydride groups.
According to DE-A 25 53 593, water-dilutable polymer oil imides are obtained by reacting polybutadiene/maleic anhydride adducts with ammonia or urea.
In DE-A 27 54 733 or 26 16 591 maleic anhydride-containing polybutadienes are reacted with diamines which include one primary or one tertiary amine function. While the primary amine function undergoes an imidation reaction with the anhydride groups, the tertiary amine function can be neutralized with acids so that the polybutadienes thus functionalized likewise become soluble in water.
Polybutadiene oils having a high cis double bond content are advantageous as a base for paint binders because of the good drying properties in air and the outstanding chemical resistance of the paint films. An improvement in the drying properties, especially the initial physical drying of polybutadiene-based binders, can be achieved, according to DE-A 27 33 274, by a graft reaction with unsaturated monomers.
Grafted polybutadiene/maleic anhydride adducts of this kind can be made soluble or dispersible in water in the same way as described above; see DE 26 33 262, 27 33 274, 27 54 733 and EP 0 170 184.
Graft reactions of unsaturated monomers with polybutadiene/maleic anhydride adducts are carried out in solution or, preferably, as a bulk polymerization with free-radical initiators. Such free-radically controlled graft polymerizations generally proceed in an irregular and incomplete way, with the consequence that significant residual amounts of monomers and oligomers remain in the binder.
In aqueous dispersions or coating materials prepared from grafted binders, these monomer and oligomer residues lead to severe odor nuisance. It is readily evident that, following the application of such dispersions or coating materials, the monomers escape from the coating film together with the water, in the course of drying, and thus produce to an environmentally polluting emission.
Moreover, unsaturated monomers customarily used for the graft reaction, for example styrene or acrylates and/or methacrylates are hazardous substances with known toxicity. Thus, a need exists for an organic solvent-free, environmentally friendly, polybutadiene-based binder which is free of odor-causing residual monomers.